A tachycardia is an arrhythmia in which the heart beats at an abnormally rapid rate, inappropriate for the tissue involved and inappropriate for metabolic need. Tachycardias usually have a rate greater than 100 beats per minute (bpm), but may be less, such as in junctional tachycardias which include any junctional rhythm faster than 60 beats per minute. Tachycardias are typically classified as being supraventricular or ventricular. Supraventricular refers to an arrhythmia whose origin is above the ventricles, such as sinus tachycardia, atrial flutter, and atrial fibrillation.
Cardiac stimulation has been utilized for many years for the purpose of terminating intrinsic atrial and/or ventricular tachycardias. An implantable stimulation device which is capable of treating tachycardias through the application of stimulation pulses, or antitachycardia pacing, is sometimes referred to as an "antitachycardia pulse generator" or an "antitachycardia pacemaker." Antitachycardia pulse generators typically are only used for supraventricular tachycardias.
If the implantable stimulation device further includes the ability to detect and treat ventricular tachycardia and ventricular fibrillation, the device is often referred to as an "antitachycardia defibrillator," an "implantable cardioverter defibrillator (ICD)," or simply a "defibrillator." Although the early defibrillators only provided shock therapy for treating ventricular fibrillation, modern defibrillators typically include both antitachycardia pacing and cardioverter/defibrillator shock therapy. As used herein, an implantable stimulation device includes any stimulation device capable of delivering stimulation pulses to disrupt an arrhythmia.
The underlying principle of using stimulation pulses to terminate a tachycardia is based on the premise that if an implantable stimulation device delivers a stimulation pulse to the heart during a critical time period following a naturally occurring heartbeat, the heart may revert to sinus, or natural, rhythm. This is best understood by reviewing the mechanism causing the arrhythmia. A tachycardia is often the result of an electrical feedback mechanism within the heart. For example, a natural heartbeat can occur through a normal pathway and re-enter through an alternate loop of tissue that perpetuates conduction (also known as an accessory or re-entrant pathway), thereby initiating a tachycardia. The delivery of a stimulation pulse causes the cardiac tissue in front of the stimulation pulse to depolarize (thereby causing the heart to contract), but leaves the tissue at the stimulation site refractory (i.e., the tissue cannot respond to additional stimulation). Thus, by injecting a stimulation pulse within the cardiac cycle, the stability of the feedback loop is disrupted, and the heart may revert to a natural sinus rhythm.
The difficulty in using a stimulation pulse to terminate a tachycardia lies in determining exactly when the stimulation pulse should be applied. It must be applied at a time shortly following one heartbeat and prior to the time when the next heartbeat would otherwise occur. On an ECG recording, this would correspond to the interval between successive R-waves.
It is known that there is usually a "region of susceptibility" within a given tachycardia cycle during which the heart is susceptible to reverting back to a sinus rhythm through the application of a stimulation pulse to the heart. This critical time period may be thought of as an arrhythmia "termination window." The terms "region of susceptibility" and "termination window" are used interchangeably herein to refer to this narrow time period.
Unfortunately, the size of the termination window (that is, the length of time available to successfully interrupt the re-entrant pathway) depends on the tachycardia rate, the length of the reentrant pathway, and the refractory periods of tissue in the pathway. The termination window may further be altered by body posture, physiological conditions, antiarrhythmic drug therapy, physical activities, diurnal cycles, catecholamine levels, and numerous other factors which affect conduction velocity and refractory characteristics of cardiac tissue. The termination window further varies not only from patient to patient, but from day to day for the same patient. Moreover, for any given patient on any given day, the termination window within the overall tachycardia cycle is relatively short and may vary even during a single tachycardia episode.
In order to increase the likelihood that a stimulation pulse will be applied during the region of susceptibility and therefore likely to be successful in terminating an arrhythmia, it is known in the art to use several techniques to "hunt" for and find the termination window. Typically, heretofore, the manner in which the "hunting" for the termination window has been accomplished, follows either one of two approaches: (1) "shotgunning" the termination window by applying a burst of stimulation pulses; or (2) "scanning" the termination window by delivering "critically timed" stimulation pulses.
The theory behind providing a burst of pulses is that sooner or later one of the stimulating pulses will occur at a time in the tachycardia cycle which will terminate the tachycardia. Burst pacing may be delivered asynchronously or synchronously at a fixed, decreasing, or increasing, cycle length. This process continues until the region of susceptibility is found, and the tachycardia is terminated. Once the region of susceptibility is found, the timing associated with the successful burst may be stored and used as the starting point for applying a new burst of simulation pulses to the heart upon the next occurrence of a tachycardia. Prior art representative of this shotgun (burst pacing) approach of terminating a tachycardia can be found in U.S. Pat. Nos. 4,398,536 (Nappholz et al.); 4,406,287 (Nappholz et al.); 4,408,606 (Spurrell et al.); 4,541,430 (Elmqvist et al.); and 4,561,442 (Vollmann et al.) .
The alternate technique to find the termination window is by "scanning." This technique utilizes an implantable stimulation device which automatically searches or "scans" for the pacing interval most likely to terminate a tachycardia. The implantable stimulation device delivers single or multiple stimulation pulses at "critically timed" coupling intervals (that is, intervals coupled to the last R-wave to terminate a tachycardia) and continues in a controlled sequence until the tachycardia terminates. For example, the controlled sequence may begin with a single stimulation pulse at one end of the scanning window and, with each successive tachycardia cycle, deliver additional pulses at increasing (or decreasing) coupling intervals in a controlled manner towards the other end of the window. Hence, the stimulation pulse scans through the scanning window looking for the region of susceptibility.
For example, in U.S. Pat. No. 4,312,356 (Sowton et al.), a pulse generator is disclosed wherein the sensing of a tachycardia triggers a stimulation pulse having a known and somewhat arbitrary timing relative to the tachycardia cycle. The stimulation pulse is thus applied to the heart at a time within the cardiac cycle that represents a first guess of the timing of the region of susceptibility. If the stimulation pulse is not successful in terminating the tachycardia, then a subsequent stimulation pulse is provided, issued either later or earlier (according to a predetermined search pattern) relative to the timing of the unsuccessful stimulation pulse. In this trial-and-error manner, the region of susceptibility is eventually located, and the tachycardia is terminated. Unfortunately, this approach may require a significant "hunting" time before the region of susceptibility is located.
In order to shorten the hunting time, it is also known in the art to store the time interval associated with the last successful stimulation pulse. This last successful time interval is then used as a starting point for the scanning window when the next tachycardia occurs. In this manner, the "hunting" time is believed to be significantly reduced. The prior art approaches using a pulse that hunts for the region of susceptibility by scanning through a scanning window can be found in U.S. Pat. Nos. 4,390,021 (Spurrell et al.) and 4,427,011 (Spurrell et al.). U.S. Pat. No. 4,577,633 (Berkovits et al.) accomplishes essentially the same result (of providing a single scanning stimulation pulse that hunts for the region of susceptibility) by continuously shortening the pulse generator escape interval with each subsequent beat, for a predetermined number of beats, by a small programmable decrement.
It is also known in the art to combine shotgunning (burst pacing) and single-pulse scanning as selectable options within a single pulse generator (U.S. Pat. No. 4,726,380 (Vollmann et al.)).
Burst pacing has been very successful in terminating cardiac arrhythmias and was initially the preferred approach. However, in some cases, burst pacing has been known to disorganize or accelerate an arrhythmia instead of terminating it. Furthermore, it is also known that the longer it takes to terminate the arrhythmia, the more difficult the arrhythmia can be to ultimately treat.
As a result, the stimulation pulse scanning techniques are currently favored in an attempt to lesson the risk of arrhythmia acceleration while maintaining a high efficacy for arrhythmia termination. Scanning of the termination window has also been necessary in order to account for the variations in the exact timing of the tachycardia termination window that regularly occurs in any given patient.
Unfortunately, however, if the "region of susceptibility" (or termination window) has shifted in a direction that causes longer intervals for termination, the starting point of the scanning sequence may require an entire scan to be completed before finding the termination window. Depending on the total range and number of critically timed intervals, the steps or amount of adjustment within each attempt, the number of pulses, and the exact sequence to be attempted, an entire cycle of scanning may be a long process.
What is needed, therefore, is an arrhythmia termination system that prevents long periods of time in locating the region of susceptibility and improves the success rate of termination, thereby minimizing the time required to terminate the arrhythmia.